processing | Optical moulding
maintaining a uniform hopper filling level (this should be considered good practice in any precision moulding application). As a general guide, material travel distances should be short and manageable, and the pel- let feeding rate should not be excessively high.
Large diameter feed channels are often required to produce high quality thick wall optical components
interfaces which can be programmed at freely defined stages of the moulding cycle. Compression moulding technology, in particular,
requires a certain degree of freedom in terms of programming, as selection of the point in time, pressure and speed of the embossing movement has a consider- able influence on the optical quality of the component. The multi-layer process is a new technology that
manufactures thick-walled lenses in several layers. Application of a final thin surface layer enables high levels of surface precision to be attained while the dwell times of the melt and cycle times can be significantly reduced. The technique can be implemented on an injection moulding machine using just one injection unit. However, as with the tandem moulding technique, it must be possible to programme sequences in such a way that several injection and holding pressure process- es take place during a cycle. (Injection World will look at multi-layer processes in more detail in a future edition)
Handling optical polymers The processing of optical materials starts with the inspection of the incoming plastic granules. Once this material has been approved and released for produc- tion, it must be subjected to proper pre-drying in terms of time and temperature. In addition, the feeding and drying systems should be dustproof and should not have an abrasive effect on the material or vice versa. Dust collection systems may be included just before the material enters the filling zone of the injection mould- ing machine to remove any particles of dirt that may be present. Particulate contamination can lead to ‘black spots’ being produced in the transparent components. Material feeding characteristics such as dosing consistency and venting can be enhanced through
38 INJECTION WORLD | September 2012
Plasticising considerations The plasticising system – which comprises the plasticis- ing cylinder as well as the screw, the non-return valve and any protection against adhesion or abrasion - must be designed for optical processing. In particular, the energy absorption and feed characteristics of the material to be processed must be considered. In many cases, the filling, compression or metering zones will need to be optimised and custom lengths, flight depths, pitches and compression ratios adopted. Optical materials tend to display adhesion or blooming, so anti-adhesion and wear protection coating selection is also important. A variety of options are available to help permanently eliminate material residue build up and subsequent release, which can also give rise to “black spots”. The non-return valve must close rapidly and
reproducibly, even when the screw decompression is very minimal or where no screw decompression is used. Some of the latest non-return valve designs guarantee complete closure after the dosing process. The closure process is independent of decompression and it is no longer necessary to retract the screw. This is particu- larly beneficial with open nozzles – which are widely used in optical moulding – as the danger of air being drawn in to the plasticising system is reduced. Air can cause clouding in optical parts. When using open machine nozzles it is important that the diameter of the nozzle bore hole is adapted to the dimensions of the mould nozzle. Diameters of 15 mm can be required with moulds for thick-walled lenses running up to eight cavities and cold runner systems. In order to retain a corresponding contact surface, these have to be kept in a flat radius. Borehole surfaces are usually polished and all edges are rounded off. Care must always be taken in the production process
that a clean, concentric nozzle fitting is maintained. Damaged nozzles, which do not fit cleanly or with borehole edges which are burred can lead to smoke- like streaks appearing in the lens due to the intake of air or melt outline effects occurring during the injection moulding process. As a basic principle, all flow channel geometries,
cross-sections and approaching surfaces should be shaped as smoothly as possible. Sharp edges, deflec- tions and dead zones must be positively avoided in the entire plasticising system.
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